1. Field of the Invention
This invention relates to a method for controlling the thickness of a skin layer of a compositely made thermoplastic resin molded product having a skin layer and a core layer.
2. Detailed Description of the Prior Art
Conventionally, when molding a composite molded product, an inner core is increased in percentage by making the viscosity of the first resin with carbon dioxide gas lower than that of the second resin (refer to the patent literature 1).
Moreover, when positively controlling a molding position of a hollow part by injecting the second molding material differing in viscosity to the first molding material, a fibrous or granular filler is added to the resin so that a suitable difference in viscosity can be obtained (refer to the patent literature 2).
Patent literature 1: The Japanese Patent Publication 1999-202694 (Page 2)
Patent literature 2: The Japanese Patent Publication 2000-141405 (Page 4)
In injection molding, the thermoplastic resin (hereafter called resin) is plasticized and injected into a metallic mold, and is solidified by cooling to mold the molded product. While most of molded products are generally molded of a single kind of resin, while it is known as sandwich molding to manufacture a composite resin molded product by injection molding it of a same or different kinds of resins.
In the case of the sandwich molding, a first resin making the skin layer forming the surface layer part of the molded product is injected into the cavity of a metallic mold in advance, and then a second resin making the core layer is injected into the first resin so as to be filled therein. Thickness of the skin layer formed by the molding depends on the viscosity characteristics of the first and second resins, and the skin layer tends to be formed thinner when the viscosity of the first resin is set lower than that of the second resin.
It is known that the viscosity of resin varies according to plasticizing temperatures and shear rates. However, the thickness of the skin layer is not freely controllable by the variation in the viscosity according to them. In addition, since there is also a fear of causing deterioration in the physical properties of the resins, the viscosity difference made at the time of molding a composite molded product is provided by artificially varying the viscosity by means of adding another material thereto.
According to the above-mentioned patent literature 1, the viscosity difference between the first and second resins is made by dissolving carbon dioxide gas in a molten resin. However, since the method using carbon dioxide gas needs not only a carbon dioxide gas supply facility but also a resin foaming machine and an arrangement for discharging and processing unnecessary used gases, the method has the problem that the arrangement costs have to be high.
Moreover, as described in the patent literature 2, in some of the means for obtaining the viscosity difference, a fibrous filler such as whiskers, metallic fiber, glass, carbon or a granular filler such as talc, calcium carbonate, mica is added to a resin. Although such means do not have the arrangement cost problem as in the case of using carbon dioxide gas, the above-mentioned filler may influence on the physical properties of the resin depending on the addition, and moreover, when the filler is added to the first resin for forming the skin layer, the filler is exposed on the surface of the skin layer, therefore, the means also has a problem that it is hard to be employed depending on molded products.
This invention has been devised in order to solve the conventional problems mentioned above, and the purpose thereof is to provide a new method by which the viscosity is varied by adding an ultrafine foreign substance to a raw material resin, and the thickness of the skin layer of the composite resin molded product is controlled by the viscosity difference arising therein, but which does not have an influence on the physical properties of the resin and the molded product by the addition of the foreign substance.